Optical components providing a given function for fiber optical communications are progressing towards smaller size with less electrical power consumption. Integrated optical technology such as planar lightwave circuits is a key enabler of this progress. The planar lightwave circuit (PLC) generally is formed on a flat substrate. Materials can be patterned to form optical waveguides that will constrain light of an appropriate wavelength range so that the light may be guided along optical pathways defined by a core material of the waveguide. Types of optical waveguides includes, but is not limited to, ridge waveguides, rib waveguides, and channel waveguides. As one example, a channel waveguide with a rectangular cross section may comprise a silica core region surrounded by a silica cladding region wherein the index of refraction of the core region is higher in value than that of the silica cladding region. In such an example, the percentage difference between to two values of index of refraction may be referred to as the index contrast. This particular example may be used to exemplify particular design considerations which may be modified to suit a particular situation by one practiced in the art. However, as known in the art, many other waveguide variations are possible and many other materials suitable for forming optical waveguides (including silicon, InP, various polymers, and various other glass materials) are possible.
Many useful functions within a PLC are provided by static devices; conversely, some functions require dynamic control from circuitry external to the PLC. A thermo-optic phase shifter is a common device that enables dynamic control of various optical functions. Typically, a thermo-optic phase shifter is formed by depositing a thin film of metal onto the top cladding above the optical waveguide. The metal film or semiconductor film can be patterned to define the boundaries of the heater, and herein is referred to as the “heater.” Desirable metal materials for heaters resist corrosion and are generally durable, such as tungsten, nickel-chromium alloys (nichrome), and other metals, such as those known in the art. Methods for forming heaters for PLCs include deposition by sputtering, other physical vapor deposition, or other suitably process followed by patterning.
One of the commercially significant devices integrated onto a PLC is a Mach-Zehnder interferometer (MZI). Providing an MZI with an optical phase shifter on one or both of the arms of an MZI can provide dynamic control over some aspects of the MZI function. A number of optical phase shifters are known in the art, including, but not limited to, thermo-optic phase shifters, carrier injection phase shifters, and carrier depletion phase shifters. The transfer function of the MZI is dependent upon the phase shift of the first arm relative to the phase shift of the second arm. In particular, the transfer function of the MZI depends on the differential phase shift and is substantially independent on the amount of phase shift that is common to both arms. When the MZI is configured with at least one thermo-optic phase shifter, the function of the phase shifter is to introduce a temperature difference between the first arm and the second arm and thereby determine the MZI transfer function. An MZI with a phase shifter on one or both arms may be used as an optical switch or, alternatively, as a variable optical attenuator (VOA).